The present invention relates to a method for operating a reciprocating positive-displacement pump for the simultaneous low-pulsation discharge of a plurality of liquids, having for each liquid at least two pump chambers and displacement devices capable of movement therein, of which the one displacement device takes in liquid during the actual discharge phase of the other displacement device, and reverses its direction of motion at the end of the intake stroke, and, in a pre-compression phase, pre-compresses the liquid taken into the associated pump chamber, and comes to a standstill when a prespecifiable pre-compression pressure is reached, and remains at a standstill until the other displacement device has concluded its liquid discharge, and, subsequent to this discharge, begins its own discharge. Moreover, the present invention relates to a reciprocating positive displacement pump for the simultaneous low-pulsation discharge of a plurality of liquids, having for each liquid at least two pump chambers and displacement devices that are capable of movement therein, of which the one displacement device takes in liquid during the actual discharge phase of the other displacement device, and reverses its direction of motion at the end of the intake stroke, and the liquid taken into the associated pump chamber is pre-compressed in a pre-compression phase and comes to a standstill when a pre-specifiable pre-compression pressure is reached, and remains at a standstill until the other displacement device has concluded its liquid discharge, and subsequent to this discharge begins its own discharge.
In known pumps of this type, the displacement devices for the individual pump chambers are rigidly connected to one another and have a common drive, usually in the form of a hydraulic cylinder.
Extraordinarily low pulsation without the use of what are known as pulsation dampers is achieved by oscillating displacement pumps having two displacement devices for each liquid, of which the one displacement device takes in liquid during the actual discharge phase of the other displacement device, and reverses its direction of motion at the end of the intake stroke, and the liquid taken into the associated pump chamber is pre-compressed in a pre-compression phase and comes to a standstill when a pressure prespecified by the system is reached, and remains at a standstill until the other displacement device has concluded its liquid discharge, and subsequent to this discharge the displacement device that is at a standstill at the end of the pre-compression phase begins its discharge.
In pumps of this type, the hydraulic pressure applied in the hydraulic drive cylinder is controlled for example by a control device according to DE 197 27 623 C1, dependent on the pressure of the current discharge side of the pump, in such a way that the pressure always remains at a safe distance below the pressure of the currently discharge side of the pump, so that liquid outlet valves of the pump chambers cannot open as a result of the higher pressure prevailing in the liquid line to the consumer.
In pumps of this type, as a rule different pre-compression pressures arise in the two discharge cylinders until the opening of one of the outlet valves, and as a rule different pre-compression pressures also occur from one discharge stroke to the next discharge stroke. It is a fairly rare coincidence for the two pre-compression pressures to be equal. The differences in the pre-compression pressures are dependent on different degrees of filling of the pump chambers, differences in viscosity, different tightnesses at the valves, differences in the compressibility of the two liquids that are to be discharged, which for example can already vary strongly from stroke to stroke merely as a result of single air bubbles, and on different degrees of elasticity of the components.
At the end of the pre-compression stroke, the hydraulic piston force of the drive cylinder is in equilibrium with the sum of the two axial forces on the displacement devices. The sum of the two displacement device forces (pre-compression pressure times surface) is always the same; i.e., the missing amount of pre-compression force at the one displacement device is added at the other displacement device. Thus, there is a double effect.
The surface ratio of the two displacement devices has a great influence on the difference in the pre-compression pressures. The greater the surface difference, the faster and farther the pre-compression pressure increases at the displacement device having the smaller surface, due to the small pump chamber volume, above the pre-compression pressure of the pump chamber having the larger-surface displacement device.
The possible pressure difference is greater:                the greater the surface difference of the displacement devices is,        the tighter a suction valve of the smaller pump chamber is,        the less tight a suction valve of the larger pump chamber is,        the faster the suction valve of the smaller pump chamber closes,        the slower the suction valve of the larger pump chamber closes,        the greater the degree of filling of the smaller pump chamber is,        the lesser the degree of filling of the larger pump chamber is,        the smaller the volume of the smaller pump chamber is,        the larger the volume of the larger pump chamber is,        the less compressible the liquid in the smaller pump chamber is, and        the more compressible the liquid in the larger pump chamber is.        
From the above, it is clear that statements about the size of the differences in the pre-compression pressures are less reliable the greater the differences in the displacement device surfaces, and the greater the differences between the two liquids that are to be discharged. For the sake of simplicity, hereinafter only different degrees of filling will be referred to.
If, given an displacement device surface ratio of 1:1, for example no pre-compression at all is achieved in one of the two pump chambers (e.g. due to insufficient filling or a leaky suction valve), the pre-compression pressure in the other cylinder increases to twice the pressure that would arise given a uniform distribution. If no pre-compression is achieved given an displacement device surface ratio of e.g. 49:1 in the cylinder having the larger displacement device, the pressure in the other cylinder increases to 49 times the pressure.
If the unequally pre-compressed pump chamber fillings of one pump side are connected by lines to the consumer, e.g. a spray gun, at the beginning of the discharge stroke, the discharge of the two liquids begins nonuniformly and with a temporal offset. The discharge cylinder having the higher pressure relative to a consumer line pressure emits, when its pressure relaxes to consumer pressure, a corresponding liquid quantity into the consumer line in pulsed fashion, in addition to the target quantity. The other discharge cylinder, whose pre-compression pressure is lower than the consumer pressure, must first be further compressed to consumer pressure before material is pressed into the consumer line. For this purpose, a displacement device stroke, i.e. time, is required. While the one pump chamber, relative to one pump side, begins with surplus discharge, the discharge from the other pump chamber begins too late, so that at the consumer there is a brief lack of liquid in the insufficiently pre-compressed discharge cylinder.
Therefore, for the present invention the problem arises of removing these disadvantages of discharge flow irregularities of the previous methods and devices, and to create a method and a device of the type indicated above that provide a discharge of the liquids that are to be discharged that is uniform at all times.